Total body magnesium (Mg) is inversely correlated with heart disease, atherosclerosis and hypertension. Serum Mg is a poor indicator of total body Mg but peripheral blood mononuclear cell Mg may be a good indicator. Peripheral blood mononuclear cells are a heterogenous population of lymphocytes and monocytes, with monocytes being twice the volume of lymphocytes. To evaluate mononuclear cell Mg, the mean cell Mg of the population, the individual cell Mg distribution within the population and the relationships between cell Mg and cell size must be determined. Whereas atomic absorption spectroscopy (AAS) methods have been developed to measure the mean mononuclear cell Mg, methods for measuring the Mg of individual mononuclear cells have not been described. We have developed a method for the quantification of Mg in individual whole mononuclear cells by electron probe X-ray microanalysis (EPMA). Whole blood is anticoagulated with heparin, washed with buffered saline glucose and ultracentrifuged in a discontinuous stractan density gradient. Isolated mononuclear cells are washed with osoosmotic, pH neutral ammonium nitrate, spray deposited on thin film supports, air dried and carbon coated for EPMA. The Mg concentration is determined by the peak-to-local continuum X-ray intensity ratio. Glass microsphere standards obtained from the National Bureau of Standards were used to calibrate the analysis. The mean cell Mg concentrations determined by AAS and EPMA were not significantly different. The coefficient of variation for replicate analysis of the standard by EPMA was 10%. The net variation in Mg concentration among individual cells was 20% and the distribution was symetrical. The Mg concentration was independent of cell size but the cell Mg content (fg/cell) increased with cell size.